Gas discharged from an internal combustion engine such as a diesel engine, a gasoline engine, or a gas turbine, that is flue gas, contains nitrogen oxides (NOx) and particulate matters (PM). Particularly, because the diesel engine burns fuel in an oxygen excessive state, nitrogen oxides (NOx) and particulate matters (PM) are contained in a large amount in the fuel. Therefore, a device that decreases particulate matters and a device that decreases nitrogen oxides are provided in an exhaust pipe of the internal combustion engine. As an example of the device that decreases nitrogen oxides, there is a device that decreases nitrogen oxides from flue gas by injecting urea into an exhaust pipe that guides flue gas, produces ammonia from urea in the exhaust pipe, causes the produced ammonia to react with nitrogen oxides in flue gas, and then removes oxygen from nitrogen oxides to produce nitrogen again.
For example, Patent Literature 1 describes a flue gas purifying device in which a DPF device and a selective catalytic reduction catalytic device are sequentially arranged from an upstream side in an exhaust path of an internal combustion engine. Patent Literature 1 also describes a device that calculates NOx emissions, at the time of a normal operation, based on an NOx emissions map for the normal operation, or at the time of forced regeneration of the DPF device, calculates NOx emissions based on an NOx emissions map for forced regeneration, to calculate a feed rate of ammonia aqueous solution corresponding to the calculated NOx emissions, and feeds ammonia aqueous solution into flue gas on an upstream side of the selective catalytic reduction catalytic device so as to reach the calculated feed rate.
Further, Patent Literature 2 describes NOx removal equipment for flue gas discharged from a combustion plant such as a waste incinerator, although it is not for treatment of flue gas from an internal combustion engine. Patent Literature 2 describes a denitration control method in which a NOx concentration in gas before treatment, an ammonia concentration in treated flue gas, a NOx concentration in flue gas, and a flow rate of flue gas are measured, to calculate a flow rate of NOx before treatment, a NOx concentration after treatment, a record of NOx removal efficiency by NOx removal equipment, and an ammonia concentration in treated flue gas based on a measurement result thereof, deviations between the calculated values and target values thereof are respectively calculated to thereby calculate correction values based on the calculated deviations, and a corrected flow rate of NOx is calculated based on at least one of the calculated correction values, thereby controlling a flow rate of ammonia to be injected into flue gas before treatment based on the calculated corrected flow rate of NOx.